The use of "metallocene" catalysts (i.e. catalysts which contain two cyclopentadienyl ligands) for the polymerization of ethylene has been investigated for some 40 years, as evidenced by a series of patents which include U.S. Pat. No. 2,827,466 (Breslow, from a filing date of Sep. 27, 1955); U.S. Pat. No. 3,231,550 (Manyik, from a filing date of Jun. 4, 1964); and U.S. Pat No. 4,542,199 (Kaminsky et al). Kaminsky et al discovered that certain alumoxanes may activate zirconium-based metallocenes in a manner that produces a "single site" catalyst with spectacular productivity--a significant discovery which led to a large increase in research in this area of catalysis. Of additional relevance to the present invention, much investigation has been done on monocyclopentadienyl transition metal catalysts which are sometimes also referred to as "pseudo-metallocenes".
When a metallocene or a pseudometallocene catalyst is employed in a slurry or gas phase polymerization, it is highly preferred to use the catalyst in a heterogeneous or "supported form". It is also highly preferred that the catalyst does not cause reactor fouling. The art of preparing heterogeneous catalysts which do not lead to reactor fouling is not adequately understood, though it is generally accepted that the catalytic material should be very well anchored to the support so as to reduce the incidence of fouling resulting from the deposition of catalyst or cocatalyst which has dissociated from the support.
In general, heterogeneous metallocene catalysts may be grouped into three main categories:
I. Unsupported Alumoxane/Metallocene Mixtures
These catalysts may be easily prepared by evaporating the solvent or diluent from a liquid mixture of an alumoxane and a metallocene. The resulting product is a solid at room temperature due to the comparatively high molecular weight of the alumoxane. There are two disadvantages to this approach, namely cost (i.e. alumoxanes are comparatively expensive--and the alumoxane is used as an expensive "support" material) and "reaction continuity/fouling" (i.e. the alumoxane may partially melt under polymerization conditions, leading to reactor instability/fouling). U. S. Pat. No. (USP) 4,752,597 (Turner, to Exxon) illustrates this approach for the preparation of a heterogeneous metallocene catalyst.
II. Metal Oxide Supported Catalysts
These catalysts are prepared by depositing the metallocene catalyst and a cocatalyst on a very porous metal oxide support. The catalyst and cocatalyst are substantially contained within the pore structure of the metal oxide particle. This means that a comparatively large metal oxide particle is used (typically particle size of from 40 to 80 microns). The preparation of this type of supported catalyst is described in U.S. Pat. No. 4,808,561 (Welborn, to Exxon).
III. Filled/Spray Dried Catalysts U.S. Pat. Nos. 5,648,310; 5,674,795 and 5,672,669 (all to Union Carbide)
teach the preparation of a heterogeneous metallocene catalyst by spray drying a mixture which contains a metallocene catalyst, a cocatalyst and a "filler" which is characterized by having a very small particle size (less than one micron) and by being unreactive with the catalyst and cocatalyst. The examples illustrate the use of very fine particle size "fumed" silica which has been treated to reduce the concentration of surface hydroxyls. The resulting catalysts exhibit good productivity. Moreover, they offer the potential to provide a catalyst which is not prone to "hot spots" (as the catalyst may be evenly distributed, at low concentration, throughout the heterogeneous matrix). However, these catalysts suffer from the potential disadvantage of being very friable because they are prepared with a fine, "inert" filler material which does not react with I anchor to the catalyst or cocatalyst.
Friable catalyst particles lead to the formation of "fines" in the polyethylene product, and may also aggravate reactor fouling problems.
It is one object of the present invention to provide a spray dried, heterogeneous metallocene catalyst which mitigates problems associated with prior art catalysts.